Born on September 1, 1923, in the north side of Brockton, Massachusetts, Rocco Francis Marchegiano was an Italian-American boxer, the World Heavyweight Champion from September 1952 to April 1956, and the only boxer to hold the heavyweight title and go untied and undefeated throughout his career. His father, Pierino Marchegiano, was from Ripa Teatina, Abruzzo, while his mother, Pasqualina Picciuto, was from San Bartolomeo in Galdo, Campania. Rocky had three sisters: Alice, Concetta and Elizabeth, and two brothers—Peter, whom they called Sonny and Louis.

He dropped out of school after finishing tenth grade, and started working first as a chute man on delivery trucks, then as a ditch digger and a shoemaker. In March 1943, he was drafted into the army for three years. While awaiting discharge, he won the 1946 Amateur Armed Forces boxing tournament. In March 17, 1947, he stepped into the ring as a professional competitor, knocking out Lee Epperson in three rounds.

Rocky returned to the amateur ranks and fought in the Golden Gloves All-East Championship Tournament in March 1948. He was beaten by Coley Wallace. He then competed in the AAU Olympic tryouts in the Boston Garden, where he knocked out George McInnis.

Rocky began fighting permanently as a professional boxer on July 12, 1948. He changed his last name into Marciano because the ring announcer in Providence, Rhode Island, could not pronounce Marchegiano, so Al Weill, his manager, suggested Rocky Mack but Rocky picked another Italian name.

He won three more fights by knockout and one by unanimous decision. Five more knockout wins followed until he fought Roland La Starza, winning by split decision on March 24, 1950. Both boxers were undefeated at the time of the fight, with La Starza's record at 37–0.

Marciano won three more knockouts in a row before a rematch with Lowry (61–56–10), which Marciano again won by unanimous decision. After that, he won four more by knockout, and, after a decision over Red Applegate (11–14–2) in late April 1951, he was showcased on national television for the first time, when he knocked out Rex Layne (34–1–2) in six rounds on July 12, 1951.

On October 27, 1951, the 28-year-old Rocky won against the 37-year-old Joe Louis, his childhood idol. After four more wins, including victories over 35-year-old Lee Savold (96–37–3) and Harry Matthews (81–3–5), he had an opportunity to win the title. He faced the World Heavyweight Champion, 38-year-old Jersey Joe Walcott, in Philadelphia on September 23, 1952. Walcott dropped Rocky in the first round, but in the thirteenth he landed a right cross and Walcott slumped to his knees with his arm draped over the ropes. He lay motionless long after he had been counted out and Rocky became the new World Heavyweight Champion. A year later, in a rematch, he knocked out Walcott in the first round. Then, he won the rematch with Roland La Starza's by a technical knockout in the eleventh round.

More champions challanged him: light heavyweight legend Ezzard Charles became the only man who lasted fifteen rounds against Rocky but he lost two fights; Don Cockell who was knocked out in the ninth round; and 38-year-old Archie Moore, who knocked down Rocky in the second round, but Rocky recovered and retained his title with a knockout in round nine.

Rocky announced his retirement on April 27, 1956. He entered the world of television, first appearing in the Combat! episode "Masquerade" and then hosting a weekly boxing show on TV in 1961. He continued as a referee and boxing commentator for many years. He was also active in business as a partner and vice president of Papa Luigi Spaghetti Dens, a San Francisco based franchise company formed by Joe Kearns and James Braly. He built a custom home at 641 NW 24 street in Wilton Manors, Florida, a suburb of Fort Lauderdale. The house still stands today.

In 1969, on the eve of his 46th birthday, Rocky was a passenger in a small private plane headed to Des Moines, Iowa. He was on his way to give a speech to support a friend's son and there was a surprise birthday celebration waiting for him. It was at night and bad weather had set in. The pilot, Glenn Belz, had only 35 hours of flying time at night, and was not certified to fly in instrument meteorological conditions. Belz tried to set the plane down at a small airfield outside Newton, Iowa, but hit a tree two miles short of the runway. Marciano, Belz and 22-year-old Frankie Farrell were killed on impact.

He is interred in a crypt at Forest Lawn Memorial Cemetery in Fort Lauderdale, Florida. His wife, who died five years after him at the age of 46, is next to him.